Free fall cone penetrometer testing (FFCPT) allows for efficient site reconnaissance and soft seabed undrained shear strength (su) assessment in a shallow water environment. The FFCPT is a particularly useful tool for near-surface characterization for cable route assessment associated with offshore renewable developments. This study presents results from an in-situ FFCPT, seismoacoustic survey and sediment coring campaign in a nearshore site in the Gulf of Finland, northern Baltic Sea. An extended FFCPT interpretation model has been applied, including friction on the shaft (as well as tip resistance), rate dependency and soil buoyancy. The derived FFCPT profiles are repeatable at a given location, while the shapes of the su profiles capture different depositional environments. The derived dynamic FFCPT su is lower than measured in the laboratory by fall cone and triaxial tests. This is potentially due to hydrodynamic drag reducing the FFCPT terminal velocity and soil drag affecting the penetration depth and dynamic su; and due to the reconstituted nature of the laboratory samples and very low stress levels being considered that are difficult to achieve in the laboratory. The magnitude of derived su and characteristic shapes in the FFCPT profiles enable ground-truthing the interpretation of seismoacoustic profiles. This study contributes to the evidence base supporting the FFCPT as a valuable supplement, or to partially replace sediment coring and laboratory measurements, for offshore site assessment.@en

This paper presents a series of laboratory free‐fall cone penetrometer (FFCP) tests conducted on marine clay samples collected from the Gulf of Finland in the Baltic Sea. Subsequently, these tests are replicated numerically with the generalized interpolation material point method (GIMP) simulations. First, the paper gives laboratory‐scale FFCP experiment results used for the validation of the numerical framework. In these experiments, a small‐scale model of a FFCP was dropped from various heights into a natural marine clay soil sample and recorded using a high‐speed camera. The tests were supplemented with a laboratory test program to determine the geotechnical properties of the clay used in the experiments. Following image processing, the tests provided data for numerical simulations: displacement, velocity, acceleration, and reaction force curves associated with the FFCP during the penetration process. The GIMP simulations shown in the paper replicate the process of penetration of the FFCP into the marine clay. The simulations used a strain‐rate dependent Tresca constitutive model, extended with strain softening that replicates the reduction of the undrained shear strength due to destructuration, an important feature of the material. The numerical simulations replicate the experiments well. The study examines the effect of cone penetrometer roughness, impact velocity, mesh density, strain rate, and strain softening on the cone penetrometer penetration process. The simulation results indicate that the presented framework can replicate the dynamic penetration process on soft and sensitive clay very well.@en